The present invention relates to a liquefied natural gas leakage testing device or calibrated source which includes a housing divided into two chambers by a gas-permeable partition.
Leakage testing devices provided with their own gas supply are used in checking and adjusting leakage indicators. A basic distinction is made between leakage testing devices in which the gas supply is in a gaseous state, e.g. helium, and liquefied, natural gas leakage testing devices in which the medium constantly flowing out from the leakage testing device is stored in liquefied form, e.g. hydrocarbon gases, in particular fluorinated and chlorinated hydrocarbon gases. In order to perform satisfactorily, leakage testing devices should have a calibrated, constant leakage rate for as long as possible, as a rule several years.
A liquefied natural gas leakage testing device is known in which the partition is formed by a membrane. The gas evaporating from one of the two chambers constantly diffuses through this membrane and thus passes to the further chamber, connected to the surroundings via a relatively large opening. The leakage rate of this prior art leakage testing device is determined by the thickness and effective cross-section of the membrane.
A particular problem in the manufacture of such leakage testing devices is to provide membranes that are suitable for this purpose, in particular for leakages of the order of magnitude of &lt;10.sup.31 6 and &gt;10.sup.-3 m bar. l/s, because it is difficult to manufacture leakage testing devices for such leakage rates since the effective membrane cross-section in the first case must be extremely small, and in the second case must be extremely large.
These difficulties would not occur if capillaries were used. However, capillaries cannot be used in leakage testing devices which employ a liquefied Freon 12 (dichlorodifluoromethane) supply. In practice it has been found that as soon as the capillaries come into contact with the liquid either they become blocked by constituents which volatize sparingly and with difficulty or the leakage rate is greatly reduced. The leakage testing device thus becomes unusable. Capillaries are therefore used only in leakage testing devices with gaseous test gas supplies, which however are not suitable for all purposes, since with leakage rates of 1.times.10.sup.-6 m bar. l/s and above the storage container would have to be unmanageably large.